Such spinal cannulas are used to puncture the vertebral canal. To make it possible to check the correct positioning of the tip of the cannula in the subarachnoidal space, it is important to optically dearly recognize and check the reflux of the cerebrospinal fluid into the cerebrospinal fluid checking chamber. In addition, secure holding of the cannula and good guiding must be guaranteed.
Numerous spinal cannulas of this class, which have a transparent grip part at the rear end of the cannula, have already been known. In some embodiments of these prior-art spinal cannulas, the cerebrospinal fluid checking chamber of the grip part is conical over its entire length, whereas in other embodiments it is conical only over approximately half the axial length, the rest being of cylindrical design. Some grip parts are provided with two diametrically opposed, recess-like grip surfaces arched uniformly over the entire length, while others have flat or stepped grip surfaces. In addition, a spinal cannula has been known, whose grip part has a plurality of radial ribs, which surround a cerebrospinal fluid checking chamber, which is conical as a whole, at regular axial distances.
Optical effects, such as reflections, refractions, and similar phenomena, which interfere with the visual checking of the reflux of cerebrospinal fluid within the cerebrospinal fluid checking chamber, occur in all prior-art spinal cannulas.
In addition, the cerebrospinal fluid checking chamber in most prior-art spinal cannulas is too short to make it possible to check the flow of cerebrospinal fluid over a sufficiently long section and time.